As a result, all Danish players from the top two tiers of the domestic league disqualified themselves from selection for the games against Slovakia and Wales, leaving the DBU with no choice but to fill their 24-man squad with second-string players from the lower leagues.

They have also included five members of the national futsal squad, which is quite possibly the first time ever that the two footballing codes have merged at full international level.

For the uninitiated, here are some of the key differences between futsal and the full 11-a-side game.

What are the basics?

Originating in South American youth clubs in the 1930s, futsal is a form of five-a-side football played on hard indoor courts between two teams of five players (four outfield, one goalkeeper) in games that consist of two 20-minute halves.

The emphasis is on entertaining, attacking football with intricate movement, skilful tricks and outlandish technique generally resulting in a higher goals-per-game average than the full-size equivalent.

How big is the pitch?

Futsal courts are about a third of the size of an association football pitch and have no boards like some five-a-side pitches. Futsal penalty areas are a semi-circle, while normal football pitches are rectangular.

How about the ball?

The official match ball is designed to aid creative play, with a futsal ball around 6cm smaller than a standard size 5 football. It's also heavier to help reduce bounce and aid players' first-touch and fleet footwork.

Are there substitute limits?

Substitutions are unlimited and "roll on, roll off," meaning that players can leave and re-enter the game as often as the coach instructs.

Any rules different?

There is no offside rule and should the ball go out of touch, throw-ins are replaced by "kick-ins" -- which, in lieu of going into too much detail, are exactly what they sound like! The ball must be kicked back into play within four seconds, while teams can also stop the clock once per half.

Are there international competitions?

Futsal is recognised by FIFA and has its own official World Cup, taking place in the even year between the World Cup proper, with Brazil dominating the competition by winning five of the eight finals played since the first in 1989.

There were 24 teams at Colombia 2016, with Argentina beating Russia 5-4 in the final. Iran came third while Portugal finished fourth.

There are also futsal versions of the Champions Leagues and Copa Libertadores that take place every season, with ex-Barcelona star Javier Saviola recently making his 100th appearance in UEFA competition in the former.

Does this mean Denmark are OK then?

Fitness is key in both sports, but whether or not Denmark's futsal stars are able to keep pace and successfully pass themselves off as convincing international calibre 11-a-side footballers remains to be seen.